I hope it's okay to bump this thread
Now that the Crystal Empire is no longer a spoiler thread, I figured I'd rewrite some of the thoughts I had in there and add another, further demonstrating I think about this show way too much
But also hopefully giving the post some reason to exist.
Others were disappointed in the lack of characterization for Sombra, I actually think the pony that needed a bit more backstory was Cadance. "The Crystal Princess returns!" What did that mean? I propose that Cadance, as only the third alicorn in the show that we've seen, and one born recently, was in fact destined to rule the Crystal Empire. Her cutie mark, as other's pointed out, glowed as she took the Crystal Heart and rallied her people.
If Celestia's 'knowledge is limited' of the Empire, it's possible she and Luna knew little of it before Sombra took over and started spreading fear and hatred literally all over Equestria. My own guess is that Sombra was a despot who overthrew the alicorn princess who first ruled the Empire. Cadance is either a forgotten descendant, a reincarnation of or somehow connected to lost Crystal Royal line. Her cutie mark and talent for spreading love indicated her destiny to rule over an empire that's job is to spread love and hope all over the land. When Celestia heard the Empire had returned, she immediately told Shining Armor and Cadance to go there. She didn't know when the Empire would return, but she would have suspect it was soon.
Now to add something else:
People have speculated that Sombra corrupted Luna, but here's another thought: The Empire reflects love all over Equestria...think of the Changlings now. Couldn't you see a swarm of changlings just hanging out in the love equivalent of Equestria's magnetic belts, "tasting the rainbow?" Imagine they are doing that before Sombra. Suddenly, they're getting a mouthful of hate...then nothing. Desperate, they grow corrupted, feasting on the love from other ponies, growing mindless, never truly satisfied.
Few changlings or ponies remember how they once lived. Chrysalis knows though, and she knows that if her subjects realize the Empire's fountain of free love is back...her iron clad power of her people as the source of finding food for them is over.